1. Field of the Invention
The present invention relates to an electrolyte-electrode joined assembly which is constructed by interposing a solid electrolyte between an anode and a cathode, and a method for producing the same.
2. Description of the Related Art
A cell, which is provided with an electrolyte-electrode joined assembly comprising a solid electrolyte interposed between an anode and a cathode, is one type of an electrochemical cell such as a fuel cell and an oxygen sensor. In such electrochemical cells, the solid electrolyte is interposed between the anode and the cathode.
In the fuel cell, materials for the anode include, for example, a cermet containing Ni and stabilized zirconia (YSZ) doped with about 8 mole % of Y2O3 in a weight ratio of 1:1. Materials for the solid electrolyte include, for example, YSZ, and examples of materials for the cathode include a perovskite type oxide such as LaMnO3.
The electrolyte-electrode joined assembly as described above is manufactured as follows. At first, NiO powder and YSZ powder are blended in a weight ratio of 1:1, followed by being pulverized and mixed with a wet ball mill or the like to prepare a paste.
Subsequently, the paste is formed into a film by the doctor blade method or the like. Thus, an anode composed of the mixture of NiO and YSZ is prepared.
Meanwhile, YSZ powder is pulverized with a wet ball mill or the like to prepare a paste. The paste is formed into a film by the doctor blade method or the like in the same manner as described above. Thus, a solid electrolyte composed of YSZ is prepared.
The solid electrolyte manufactured as described above is stacked on one end surface of the anode, and the anode and the solid electrolyte are simultaneously sintered in this state. Accordingly, the anode and the solid electrolyte are joined together.
Subsequently, a paste of LaMnO3 is applied to one end surface of the solid electrolyte to form a film, for example, by the screen printing method. When the paste is heated together with the solid electrolyte and the anode, an electrolyte-electrode joined assembly is obtained, in which the cathode is fired on the solid electrolyte, and the solid electrolyte is interposed between the anode and the cathode.
In a fuel cell provided with the electrolyte-electrode joined assembly as described above, a fuel gas containing hydrogen is supplied to the anode, while an oxygen-containing gas containing oxygen is supplied to the cathode. In particular, the fuel gas is moved toward the solid electrolyte through pores of the anode. The oxygen contained in the oxygen-containing gas combines with the electron in the cathode to produce oxide ion (O2−). The hydrogen contained in the fuel gas combines with the oxide ion (O2−) having arrived at YSZ contained in the anode from the cathode via the solid electrolyte to produce steam and electrons. The steam is moved to one end surface of the anode through pores of the anode, and the steam is finally discharged from the one end surface.
As described above, the anode is required to be a porous member including pores at a predetermined ratio in order to quickly diffuse the fuel gas supplied to the anode and the steam which is produced. However, the anode provided in accordance with the method as described above, generally has a small porosity. Therefore, the fuel gas and the steam are slowly diffused, and it is difficult to facilitate the cell reaction. In this case, the overvoltage is increased, and the cell voltage is decreased when the current density is increased.
In particular, the steam has a large molecular weight and a large specific gravity as compared with hydrogen. Therefore, the steam tends to remain in the pores. When the remaining amount of the steam is increased, it is difficult for the hydrogen to be diffused. Therefore, it is difficult to facilitate the cell reaction.
Japanese Laid-Open Patent Publication Nos. 10-64556 and 6-163055 suggest increasing the porosity by adding a pore-forming agent to a paste or particles as raw materials for an anode. However, even if the porosity is increased, it is difficult to suppress the accumulation of the steam. Therefore, it is difficult to facilitate the cell reaction to increase the cell voltage.